The ssl session contains all information required to re-establish the
connection without a new handshake.

SSL_get0_session() returns a pointer to the actual session. As
the reference counter is not incremented, the pointer is only valid while
the connection is in use. If SSL_clear(3) or
SSL_free(3) is called, the session may be removed completely (if considered bad), and
the pointer obtained will become invalid. Even if the session is valid, it
can be removed at any time due to timeout during SSL_CTX_flush_sessions(3).

If the data is to be kept, SSL_get1_session() will increment
the reference count, so that the session will not be implicitly removed by
other operations but stays in memory. In order to remove the session
SSL_SESSION_free(3) must be explicitly called once to decrement the reference count again.

SSL_SESSION objects keep internal link information about the session cache
list, when being inserted into one SSL_CTX object's session cache. One
SSL_SESSION object, regardless of its reference count, must therefore only
be used with one SSL_CTX object (and the SSL objects created from this
SSL_CTX object).